1. Field of the Invention
The present invention relates to a pixel circuit for a global shutter of a substrate stacked image sensor, and more particularly, to a pixel circuit for a global shutter of a substrate stacked image sensor which does not include a reset transistor of a photodiode, prevents noise caused by a gap between transistors, and has a substrate-stacked 3D structure.
2. Description of the Related Art
In general, a pixel circuit of an image sensor, to which a CMOS image sensor (CIS) is applied, performs a rolling shutter operation and a global shutter operation. In the case of the rolling shutter operation, signals which are photoelectrically converted by photodiodes of each row within one frame are transmitted to floating diffusion nodes by one row which is sequentially selected, and an image signal of a corresponding pixel is outputted. In the case of the global shutter operation, the entire signals which are photoelectrically converted by all photodiodes within one frame are transmitted to floating diffusion nodes at a time, and an image signal of a corresponding pixel is outputted from a row which is sequentially selected.
Recently, as a method for improving the integration degree of an image sensor, a substrate-stacked image sensor has been developed, in which two semiconductor chips and a corresponding pad (node) are stacked to be electrically coupled to each other.
FIG. 1 illustrates a cross-sectional structure of a substrate which includes a photodiode and transistors of a pixel circuit for a global shutter of a conventional substrate-stacked image sensor. Specifically, the substrate includes a photodiode PD, a reset transistor Rx, a first transfer transistor Tx1, a memory transistor Mx, and a second transfer transistor Tx2.
Electric charges generated through a light sensing operation of the photodiode PD are outputted to a floating diffusion node FD through the first transfer transistor Tx1, the memory transistor Mx, and the second transfer transistor Tx2.
FIG. 2 illustrates that electric charges (or electrons) are transferred through the first transfer transistor Tx1, the memory transistor Mx, and the second transfer transistor Tx2.
Referring to FIGS. 1 and 2, a gap (noise occurrence region) exists between the first transfer transistor Tx1 and the memory transistor Mx and between the memory transistor Mx and the second transfer transistor Tx2. As illustrated in FIG. 2, a flow of electric charges cannot be controlled at the gap. Thus, the transfer efficiency of the electric charges is degraded. Furthermore, noise may occur at the gap.
In a reset mode, the reset transistor Rx is turned on, and the photodiode PD is coupled to a power supply terminal VDD through the reset transistor Rx and reset to the voltage level of the power supply terminal VDD.
As such, the global shutter device using the conventional substrate-stacked image sensor uses a separate reset transistor to reset the photodiode. Thus, since the reset transistor occupies a space, there are difficulties in implementing a pixel having a smaller size.
Furthermore, a flow of electric charges cannot be controlled at the gaps between the first transfer transistor, the memory transistor and between the memory transistor and the second transfer transistor. Thus, the transfer efficiency of electric charges may be degraded, and noise may occur at the gaps.